Kinematic Properties of the TW Hya Association

Abstract

A kinematic analysis of the young stellar association TW Hya has been performed. The components of the displacement matrix in the Ogorodnikov–Milne linear model have been estimated both graphically and by solving the basic kinematic equations. The association’s volume expansion with the coefficient \(K_{xyz}=103\pm 9\) km s\({}^{-1}\) kpc\({}^{-1}\) was confirmed, which yields a dynamical age estimate of \(t=9.7\pm 0.8\) Myr. Using the graphical method, estimates of the association proper rigid-body rotation parameters \(\omega\) around the galactic axes \(x\) and \(y\) have been obtained for the first time, with velocity values in the range of 50–70 km s\({}^{-1}\) kpc\({}^{-1}\) and errors in their determination of 14–19 km s\({}^{-1}\) kpc\({}^{-1}\). However, these values are not confirmed by another method. For example, when solving kinematic equations using only proper motions, all three components of the rigid body rotation do not differ significantly from zero, \((\omega_{x},\omega_{y},\omega_{z})=(4,7,11)\pm(5,5,5)\) km s\({}^{-1}\) kpc\({}^{-1}\).